Sunday, 26 June 2011

Two Great Geological Words

It's time for another Accretionary Wedge, and Evelyn has asked for our favourite geological words. I briefly toyed with mentioning all the rude words and phrases (my AS Biology lads are rather taken with the thought of entering a profession where they can say "fantastic cleavage and a significant prospect of bedding" without being fired, slapped or both). My own dear husband gets far more mileage out of "schist" and "gneiss" than any non-geologist should.

Instead, I went with two words with similar etymology: "turbidite" and "bioturbation". Both stem from the latin noun turba, meaning "disturbance". Now, turbidites were some of the first aspects of geology I remember studying, and I'll confess to not having a bleeding clue what the lecturer was going on about until much later. I had some turbidite sequences in my geological mapping area, but no beautiful Bouma sequences.

SEPM have an excellent page describing the fining-up nature of a Bouma sequence. When one finds it in the field it is an indicator that these rocks were once on a continental slope, hundreds of metres below the surface of the ocean. A mind-blowing thought for any first-year undergrad in geology.

I made a pseudo-Bouma sequence with some of my BTEC students a couple of weeks ago (although not in the context of geology). We did the old soil jar test. Sure enough, when we'd given our jar of soil and water a good old shake, the largest clasts settled out first. The silt settled within an hour, but the clay actually took well over two weeks to completely settle out of the water.

Yes, that is my dinosaur ruler, bought from the Natural History Museum nearly 30 years ago.

Bioturbation is a geological event that is a lot easier to see as it happens. Thousands of organisms make their homes within the sediment, and most of the rest exist on top of the sediment, whether on land or underwater. Their burrows, footprints and traces churn up sediment and leave sometimes an indelible mark in the rocks. I love a good dinosaur trackway, I do, and it's great to see them in their most recognisable orientation:

But in cross-section the story would be very different. There are three claws that dug into the sand, distorting the layers underneath. The pressure of this bipedal dinosaur would have been enough to compress layers under its footprint, like these footprints from the Mammoth Site.

I spent a very happy summer working at the Mammoth Site, giving tours of the bone bed, and can remember these exact footprints from the second stopping point of the tour. It was a great way to introduce how the mammoths got trapped - the sediment is all messed up from the suction generated by the mammoths trying to move their feet in the mud.

All through my geology degree it was a race to get to the outcrop before the lads tried climbing it, as it would invariably be subjected to some post-lithification bioturbation, accompanied by the shrieks as they slid down the rock face. So I have fond memories of my fellow geologists wrecking the outcrops for the next year group as well.